I have the same problem. My files is originally 600 MB and lots of our clients are angry that it is corrupted. The problem is that when they download it via WP Download Manager plugin, if browser starts the download, it will stop at exactly 500 MB. But if they start the download with IDM, it downloads the file intact. I tested it many times and it seems that WPDM only allows each connection to download up to 500 MB and since IDM breaks the download in 8 connection, it’s fine.
I played with every parameter in WPDM settings and nothing solved this problem.
And I’m sure that there is no problem with WordPress or server because I can download the file with browser from direct link.
I would be so thankful if you could give us a working solution.